The survivability of ships, modern military aircraft and other air vehicles such as missiles and satellites, and land-based vehicles may be traced, at least partially, to the relative observability of the vehicle to enemy detection systems. Principally, enemy detection systems rely on reflected radar waves and emitted infrared radiation to acquire and target military vehicles. Stealth technology and other methodologies were developed to defeat this acquisition and targeting capability by reducing a vehicle's overall radar cross section and infrared emissions. In response, counter-stealth technology was developed which yielded more precise detection systems that could isolate specific vehicle components that dominated an otherwise benign radar cross section. Accordingly, vehicles could be identified and targeted based on the signature produced by a few highly observable components, despite a relatively reduced radar and infrared cross section.
Aircraft, ship, and land-based vehicle propulsion systems project a unique, highly observable and often-targeted component signature. Perhaps the most observable element of many propulsion systems is the air inlet ports necessary to provide these systems sufficient quantities of air. Air is used by propulsion systems generally for combustion or cooling. The air supplied by inlet ports may also be used to sustain onboard systems such as Heating Ventilation and Air Conditioning (“HVAC”) systems and the like.
Air inlet ports generally include a cavity that extends inwardly into the exterior surface of a vehicle to define a channel or duct for delivering air to the onboard systems referenced above. Impinging radar signals reflect off of the basic cavity/duct geometry of conventional air inlet ports to produce a distinctive radar reflection pattern, or signature, that can be identified and possibly targeted by enemy detection systems.
Apart from providing a window to wave reflecting surfaces or cavities, air inlet ports also provide a source of emitted infrared radiation which also may be used to identify and subsequently target a vehicle. Infrared radiation extends out from a heated or cooled source in a generally spherical or lambertian fashion. Moreover, enemy detection systems rely, at least in part, on their ability to locate areas of infrared radiation contrast (i.e., areas which are warmer or cooler than surrounding areas) when acquiring a target. Accordingly, air inlet ports are preferably designed to prevent line-of-sight interrogation of hot internal components or the relatively cooler surrounding condensation surfaces which could provide a distinctive infrared contrast area for acquisition by enemy detection systems.
Conventionally, louvers and/or screens have been provided within the openings defined by air inlet ports in order to reduce a port's radar and infrared signature while also preventing the ingestion of foreign matter. As detection systems have become more precise, it has become evident that louvers and/or screens present several problems. For example, to be effective in a low signature environment, both louvers and screens should be treated with radar suppressing materials. For louvers and especially screens, such treatments must be relatively thin to prevent blockage of the air inlet port. Unfortunately, in many cases the design of the louver and screen requires a radar coating so thin that its overall effectiveness is limited. This is especially true for impinging radar waves having relatively low frequencies on the order of 2 GHz.
Another problem associated with louvers and screens is the fact that each is supported within the air inlet port by a frame member. The border defined by the intersection of the louver/screen and the frame member forms a potential scatter center for radar waves. Accordingly, significant attention and expense must be allocated to ensure that such borders do not produce highly observable dihedral and trihedral structures as known to one of ordinary skill in the art.
Finally, screens in particular present a significant problem. Air inlet screens must be sized such that no radar energy can penetrate their mesh. This results in a reflective surface for radar energy and further generates surface noise due to the wire geometry. In addition, this mesh configuration makes it difficult to provide adequate infrared signature treatment and reduces the air flow efficiency of the inlet port.
Accordingly, it is desirable then to produce an improved apparatus for disguising an air inlet port from radar and infrared detection. Further, it is desirable to produce an improved apparatus which is capable of receiving an effective surface coating of radar and infrared radiation suppressing materials without significantly limiting the effectiveness of the air inlet port.